In view of the expected decrease for the next years of the heavy fractions in the oil demand, as a result of the development of new primary energy sources, cracking gasolines will constitute in the future a substantial part of the motor gasolines.
As compared with reformates and alkylates, the octane numbers of cracking gasolines are relatively low (clear RON=90-92, clear MON 79-80). Since, moreover, the European regulation provides for a decrease of the tetraalkyl lead content of gasolines, in a first stage to 0.15 g Pb/1, and will subsequently forbid any lead content in a part of the motor fuels, it seems that the octane specifications of premium gasoline will be difficult to comply with when using cracking gasolines.
In order to meet with these requirements without too much increasing the severity of the refining operations, other means shall be found to increase the octane quality of these gasolines.
Generally, these cracking gasolines, for sweetening requirements, are separated into light and heavy fractions. The light cracking gasolines contain a substantial amount of olefins, for example 30-40%, including tertiary olefins, isoamylenes, isohexenes and isoheptenes.
As for isobutene, these tertiary olefins may be converted to ethers by reaction with an alcohol, such as methanol, in the presence of an acid catalyst. This etherification has the following advantages : increase of the octane numbers of gasoline, due to the fact that ethers have higher octane numbers than the olefins from which there are formed; decrease of the gasoline olefin content, upgrading of methanol to premium gasoline without the difficulties arising when using free methanol.
The more usual ethers are MTBE (methyl tert-butyl ether) and TAME (tert-amylmethyl-ether).
As it is known, the reaction between methanol and tertiary olefin is balanced and hence it is difficult to obtain high conversion rates. On the other hand, the equilibrium is the less displaced in favor of ethers formation as the molecular weight of the olefin is higher. Thus, whereas it is possible for isobutene to obtain, by mere passage over the catalyst, conversion rates of about 93 to 98%, for isoamylenes, the conversion rates remain in the range of about 65-75% when the use of a large methanol excess is not desirable.
The residual methanol content at the output of the reactor is another important problem inasmuch as the introduction of free methanol in a motor-fuel is not allowed, except in the presence of a co-solvent, such as t-butyl alcohol.
Hence, when it is desired to obtain acceptable conversion rates of tert-olefins, it becomes necessary to use a very large methanol excess with respect to stoichiometry; but, in these conditions, the methanol amount contained in the effluent is too high for being easily removed by conventional processes such as azeotropic distillation with hydrocarbons and recycling to the reactor, as described in the French Pat. No. 2 411 881.
When all the methanol is to be removed by azeotropic distillation, as described in the above-mentioned patent, then the molar ratio methanol/olefins, at the reactor input, must be limited. The recycling to the reactor of a part of the azeotropic pentanes pentenes methanol distillate as described in the above-mentioned patent, provides only for a relatively low isoamylenes conversion of about 70%.